1. Field of the Invention
This invention relates to a brushless DC motor using a giant magnetoresistive element as a magnetoelectric converting element which detects magnetic poles of a magnetic rotor.
2. Description of the Prior Art
In recent years, brushless DC motors are widely used as a capstan motor for forwarding a tape in a videotape recorder (VTR) or an audiotape recorder, a spindle motor for driving a floppy disk or a compact disk, and the like. Likewise in the field of such applications as factory automation (FA) and robot, brushless DC motors which are smaller in size and larger in torque are used.
A conventional brushless DC motor, as illustrated in FIG. 1, is composed of a magnetic rotor (hereinafter referred to as xe2x80x9crotor magnetxe2x80x9d) 10 formed of a permanent magnet and accommodated rotatably in a housing, exciting oils 11 disposed around the rotor magnet 10 as opposed thereto across a prescribed gap, and rotational angle detecting magnetic sensors 12 interposed between the exciting coils 11. The rotor magnet 10 is rotationally driven by orderly exciting the exciting coils 11 externally depending on the position of magnetic poles of the rotor magnet and the speed of rotation can be controlled with the signal of excitation. Heretofore, a Hall sensor has been employed as the magnetic sensors for discriminating N-S poles of the rotor magnet 10.
An example of the conventional brushless DC motor is shown in FIG. 2.
In FIG. 2, a cylindrical frame 13 is provided at its lower end with a flange part 14 formed integrally therewith, on which a stator core 15 is set in place through the medium of a spacer 16. A circuit substrate 17 including a driving circuit, etc. is also placed on the flange part 14 and fixedly secured thereto together with the stator core 15 and the spacer 16 with screws 18. The stator core 15 is provided at its outer periphery side with a plurality of projected poles on which the exciting coils 11 of each phase are wound respectively. A rotor magnet 10a is disposed around the stator core 15 in such a manner that the inner peripheral surface of the rotor magnet is opposed to the outer peripheral surface of the stator core across a suitable gap. The rotor magnet 10a is fixedly secured to the inner peripheral surface of a rotor casing 19 having generally dish-like contours and magnetized so as to possess a plurality of magnetic poles. The center portion of the rotor casing 19 is fixedly secured to a boss 21 into which a rotating shaft 20 is fitted. The rotating shaft 20 is rotatably supported by the upper and lower pairs of ball bearings 22 and 22 which are arranged inside the frame 13 mentioned above.
Three Hall elements 12 connected to the circuit substrate 17 are respectively disposed in the proximity of the rotor magnet 10a so as to face to the lower surface thereof (see FIG. 1). The Hall elements 12 detect the magnetic poles of the rotor magnet 10a and the detection output is inputted to the aforementioned driving circuit, which in turn passes the electric current through the exciting coils 11 of respective phases at proper time intervals. Thus, by passing the electric current through the exciting coils 11 according to the position of the magnetic poles of the rotor magnet 10a, the rotor magnet 10a is urged to rotate as well known in the art. To each of three Hall elements 12, four sensor leads, i.e. two current input terminals and two output terminals are connected, these terminals being connected to a motor driving circuit through the medium of exciting coils 11 of the stator core 15.
In the brushless DC motor the rotation of the magnets constituting the rotor is detected and the output signal is used as a rotating signal for controlling the rotation of motor. To this end, the sensor is required to have responsiveness to a magnetic field of several kOe and the ability to discriminate the polarity (N/S). Further, it is required to possess the high frequency characteristics in order to detect the N/S poles of the rotor magnet which rotates at high speed. The Hall element has been heretofore used as a sensor which satisfies these requirements. When the electric current flowing through the exciting coil is increased to obtain high output power, however, the sensitivity of the Hall element using a semiconductor such as InSb which is unstable at an elevated temperature decreases because a temperature of the environment in the motor is elevated. This poses the problem that the Hall element is unserviceable at elevated temperatures and the sensor determines the output of the motor. Moreover, since the Hall element requires four lead wires, two for input and two for output, and three elements are usually used in one motor, it is necessary to use twelve lead wires in total. As a result, the layout of wires is complicated, which gives the largest cause to prevent the miniaturization of the motor.
To overcome the problem mentioned above, the idea of utilizing a magnetoresistive element in place of the Hall element is proposed in published Japanese Patent Application, KOKAI (Early Publication) No. 5-207721 and No. 6-245464, for example.
The term xe2x80x9cmagnetoresistance (MR) effectxe2x80x9d as used herein means a phenomenon that the electric resistance offered by a given material is varied by applying a magnetic field to that material. Generally, a ferromagnetic material is used as an MR element. A CoFe alloy having a rate of change of about 5% and a permalloy having a rate of change of about 2%, in magnetoresistance, are typical examples of the MR element. The rate of change of the magnetoresistance effect (magnetoresistance ratio, MR ratio) is expressed by the following formula (1):
Magnetoresistance ratio(%)=[R(O)xe2x88x92R(H)]/R(O)xc3x97100xe2x80x83xe2x80x83(1)
wherein R(O) represents the electric resistance in the absence of a magnetic field and R(H) represents the electric resistance in the presence of application of a magnetic field.
The utilization of the magnetoresistance effect is effective in realizing miniaturization of a motor as by reducing the number of necessary sensor leads to two and simplifying the layout of wires, for example. Since the brushless DC motor uses a magnet (having a surface magnetic field of not less than 100 [Oe]) as a rotor thereof and the exciting coil thereof for driving the rotor has a strong magnetic field (some hundreds of Oe), however, the sensor which utilizes a magnetoresistive element formed of a soft magnetic material represented by permalloy has the problem that it cannot detect the rotational angle because its detectable magnetic field (not more than some tens of Oe) is surpassed. Further, the magnetoresistive element made of an alloy can not be used under the environment in which the magnetic fluctuation is large, because the magnetic range to which it can respond is narrow.
The magnetic sensor for use in the small brushless DC motor described above is required to fulfill the following four requirements. The sensors of the conventional class, however, have the problem that none of them cannot satisfy all these requirements.
(1) The sensor should be capable of being easily miniaturized (miniaturization).
(2) The sensor should not suffer the detecting sensitivity of the magnetic field thereof to vary notably with temperature (temperature characteristics).
(3) The sensor should be capable of detecting a magnetic field of up to several kOe (magnetic field characteristics).
(4) The sensor should be capable of detecting an AC magnetic field of up to several kHz (frequency characteristics).
To satisfy these requirements, the present invention provides a brushless DC motor which comprises a stator provided with a plurality of coils, a rotor magnet magnetized so as to possess a plurality of magnetic poles and rotatably disposed as opposed to the coils of the stator, and a sensor capable of detecting the magnetic poles of the rotor magnet, in which the electric current supplied to the coils of respective phases is controlled based on a detection signal from the magnetic sensor so that the rotor magnet is rotationally driven, and the magnetic sensor comprises in combination a giant magnetoresistive element disposed as opposed to the rotor magnet and a magnet disposed on the rear side of the giant magnetoresistive element.
The giant magnetoresistive element mentioned above is formed of a magnetic particle dispersion type giant magnetoresistive material, an artificial lattice type giant magnetoresistive material, or a colossal magnetoresistive material and is capable of reading an AC magnetic field of at least not less than 5 kHz as a signal.
Preferably, the giant magnetoresistive element mentioned above is a magnetic particle dispersion type giant magnetoresistive element which is formed of a material having ferromagnetic particles, about 1 nm to about 500 nm, preferably 1 to 100 nm in maximum major diameter, dispersed in a nonmagnetic (paramagnetic or diamagnetic) material, preferably in such a manner that respective magnetic particles are dispersed therein as separated from each other without contacting at a distance of about 0.5 nm or more, so that the element has the sensitivity to a magnetic field, even under the higher magnetic field of about 50 [Oe] or more in the motor, and the responsiveness to a change in magnetic field of at least 10 kHz in the motor. It may be otherwise an artificial lattice type giant magnetoresistive element which is formed of a material having a nonmagnetic (paramagnetic or diamagnetic) material and a ferromagnetic material, each having a thickness of about 1 nm to about 10 nm, for example, alternately superposed.
By using as a magnetic sensor for a brushless DC motor the giant magnetoresistive element, particularly the magnetic particle dispersion type giant magnetoresistive element in conjunction with a magnet for the application of a bias magnetic field, the magnetic sensor can acquire the wide sensitivity to an AC magnetic field as well as heat resistance, responsiveness to a high frequency, and the sensitivity to a high magnetic field of at least 1 kOe and can detect a change in magnetic field under the environment of a strong magnetic field, and the miniaturization of sensor can be realized. Further, since these giant magnetoresistive elements retain the magnetoresistance effect even at such high temperatures as 300xc2x0 C., they can be used effectively in an atmosphere retained at elevated temperatures. Accordingly, since the maximum service temperature can be increased to at least 200xc2x0 C., the electric current to be passed through the exciting coils can be considerably increased, which results in the increase of the rotational power of the motor. That is to say, it can be expected that the brushless DC motor will be widely used in various applications by virtue of the increase of the service temperature and the rotational power of the motor.